feat(reminders): show an in-app card with done and snooze when a reminder fires

[kYem]

Fixes ENG-140

• A fire adds a device-local prompt (reminderPrompts, not synced: each device fires its own wakes). Every open tab watches it; only the focused, visible tab shows the card, above the bell.
• Done and Snooze (5/15/30m) go through respondToReminder, which the notification buttons now use too, so both behave identically. Answering anywhere clears the notification and the prompt, so other tabs' cards go away. Moving that code out of the worker also makes the notification's Done/Snooze call notifyMutated, which it didn't before.
• × dismisses the card only; the reminder stays in Needs response. Done, Snooze or Delete from the bell panel also clear the card.
• The no-background fallback raises the card instead of the plain toast, and toasts only if the prompt can't be stored.

Decisions:

• Notifications setting: enableNotifications is the Pomodoro session switch and reminders deliberately ignore it, so the card always shows.
• Focus Mode: the card lives with the bell on the new-tab home page, while Focus Mode lives on the Pomodoro page, so a reminder firing mid-session waits there as a card until you're back.
• Open bell panel: the card steps aside while the panel is open, since the panel already shows the reminder.

Tests: unit tests for the prompts list, respondToReminder, the store and the card; a new real-extension e2e reminder-prompt-card.spec.ts (fire in the worker → card on the page → Done completes it and closes the notification); all 10 extension e2e specs pass. Screenshots in all four themes were checked locally.